In radio communications systems, it is known to employ cellular base station transmitters outputting signals with different carrier frequencies. In such transmitters, it is of great importance to be able to control the radio frequency gains and accordingly the output powers for each carrier accurately to predetermined levels.
In conventional base station transmitters, which comprise a separate transmitter for each carrier, it is possible to determine the radio frequency gain for each carrier independently from the gain of the other carriers.
For illustration, FIG. 1 shows a block diagram of such a conventional base station transmitter based on RF (radio frequency) IQ (in-phase and quadrature) modulators. The base station transmitter comprises N single-carrier transmitters, of which the first one and the last one are shown. Signs of components or of values of the transmitters having an index 1 or N indicate that they are assigned to the 1st or Nth single-carrier transmitter.
Each of the N single-carrier transmitters includes a baseband modulator 1 connected at its input to elements (not shown) of a communications network supplying data symbols and at its outputs to two digital-to-analogue converters 3, 4. The digital-to-analogue converters 3, 4 are connected to inputs of an RF modulator 5. An additional input of the RF modulator 5 is connected to a local oscillator (LO) 6, while the output of the RF modulator 5 is connected to an input of a variable gain RF amplifier 7. The output of the RF amplifier 7 is connected to a single carrier power amplifier (SCPA) 8 and the output of the SCPA 8 of each single-carrier transmitter is connected via a common summation unit 10 to a transmit antenna 11. The output of the SCPA 8 is further connected to an input of a power detection and control unit 9 belonging to the respective single carrier transmitter.
Equally, the baseband modulator 1 is connected via a baseband power detection unit 2 to an input of the power detection and control unit 9. The output of the power detection and control unit 9 forms a gain controlling input of the RF amplifier 7. In practice, there can be included more upconversion stages and amplifiers, and filters may be included as well.
The baseband modulators 1 of the N single-carrier transmitters receive symbols from the network that are to be transmitted via the transmit antenna 11 over the air interface. The baseband modulator 1 of the respective transmitter generates a digitised signal trajectory in the complex plain in IQ format and forwards the signals to the two digital-to-analogue converters (DAC) 3, 4. Each of the digital IQ signals is converted into an analogue signal I, Q by one of the two digital-to-analogue converters 3, 4 and then fed to the RF modulator 5. In the RF modulator 5, both signals I, Q are modulated onto one of N carriers determined by the local oscillator 6 associated to the respective single-carrier transmitter. The output signal of the RF modulator 5 is then amplified by the RF amplifier 7 according to the gain set according to a gain control signal GC1, GCN applied to the respective RF amplifier 7, and fed to the SCPA 8. The powers output by the N single-carrier transmitters are combined at the output of the SCPA 8 by the summation unit 10 for transmission by the transmit antenna 11.
The power REF1, REFN of the output signal of each baseband modulator 1 is computed in the associated baseband power detection unit 2 and forwarded to the respective power detection and control unit 9. Equally, the output of each of the SCPAs 8 is fed additionally to the respective power detection and control unit 9, where the output carrier power is measured and compared to the output power provided by the baseband power detection unit 2 of the corresponding single-carrier transmitter. The quotient of these powers constitutes the gain of the respective RF path, G1, GN. If the measured gain G1, GN on the RF path of one of the N single-carrier transmitters deviates from the desired value, the responsible power detection and control unit 9 changes the gain control signal GC1 GCN applied to the respective RF amplifier 7 for this path in order to steer the gain G1, GN into the direction of the desired gain.
Equally, an independent power control of the different carriers is possible in another embodiment of a conventional base station transmitter shown in FIG. 2. The base station transmitter corresponds to the one of FIG. 1, except that each baseband modulator 1 is now connected to the respective RF amplifier 7 via a digital upconverter 12 and a single digital-to-analogue converter 14. An input of the digital upconverter 12 is further connected to a numerically controlled oscillator (NCO) 13. To the components of the single carrier transmitters corresponding to the components of the single carrier transmitters of FIG. 1, the same reference signs were assigned.
In contrast to the example of FIG. 1, here the conversion of the digital IQ signals output by one of the baseband modulators 1 to a modulated RF signal is carried out in the digital domain by the respective digital upconverter 12, the frequency of which is determined by the NCO 13 associated to the digital upconverter 12. The output of the digital upconverter 12 is then converted to an analogue signal by the single digital-to-analogue converter 14. Presently, digital-to-analogue converters 14 are not capable of generating high quality signals at GHz frequencies. Therefore, the architecture of FIG. 2 has in practice at least one extra analogue upconversion stage. However, for the sake of simplicity this is not shown in the diagram.
Since the power output by the baseband modulators 1 and the output of the SCPAs 8 correspond to the outputs of baseband modulators 1 and SCPAs 8 of FIG. 1 and are fed to the power detection and control units 9 as in the example of FIG. 1, the RF gain for each carrier can be determined independently as described with reference to FIG. 1. Again, gain control signals GC1, GCN are provided by the power detection and control units 9 according to the determined gains G1, GN and supplied to the respective RF amplifier 7 in order to adjust the gain for each carrier to a predetermined value.
The base station is required to control the output power used for each carrier accurately to a predetermined value. At maximum output power, the GSM (Global System for Mobile communication) and WCDMA (Wideband Code Division Multiple Access) standards demand an accuracy of better than ±2 dB per carrier. In order to achieve this accuracy reliably, the power measurement accuracy should in practice even be better than ±1 dB.
If a single carrier power amplifier is used for each carrier, this accuracy can be achieved e.g. with one of the architectures described with reference to FIGS. 1 and 2, since an access to the separate output powers of each carrier is given. Combining the carriers only at the single carrier power amplifier outputs, though, has several drawbacks. Output power is lost and changing the number of carriers in a base station takes much effort. Future base station will therefore combine the carriers already before power amplification or even earlier. The carriers are then power amplified by a single multi-carrier power amplifier. This, however, causes problems for the power control, since the individual power of the power amplified carriers cannot be accessed any more but only the multi-carrier signal output by the single multi-carrier power amplifier. Therefore, an accurate estimation of the individual carrier RF gains becomes more complicated.
In a known approach, it is simply assumed that the RF gain is equal for all carriers. Accordingly, the total output power is measured and divided by the sum of the output powers of the baseband modulators. This quotient constitutes the total gain. If the measured gain deviates from the desired value, the gain control signals for each RF amplifier are changed equally in order to adjust the gain to the right value. The drawback of this method is that there is no way to ensure that the RF gains for the different carriers are indeed all equal and will stay equal for all values of the common gain control signal, under all environmental conditions and during the whole lifetime of the base station. The relation of the gains to each other can be verified only during the assembly of the base station and, after putting into operation, by a site visit to check.
In an alternative approach, it was proposed to use a channeliser to separate the individual carriers from each other at the output of the single multi-carrier power amplifier. The powers of the separated carriers can then be measured and compared to the powers of the baseband signals. By division of the respective pair of values for one carrier, the gain for the individual carriers is found. If one of the gains deviates from the predetermined gain for this carrier, the gain can be adjusted individually by a corresponding gain control signal. The disadvantage of this method is that a channeliser is needed. The required selectivity is such that its implementation must be at an intermediate frequency or baseband. Therefore, one or two downconversion stages are needed, which increases complexity and adds uncertainty to the measurement. In practice, the power measurement circuitry moreover needs some automatic calibration circuit to maintain its accuracy. Therefore, the power control becomes rather expensive and space consuming. Moreover, in case frequency hopping transmitters are used, like e.g. in the GSM, also the channelisers have to be suited for frequency hopping, which makes the construction even more complex.